The present invention relates broadly to the field of electromagnetic control devices and, more particularly, pertains to improvements in solenoids controlled by pulse width modulation (PWM).
As is well known, a solenoid is essentially a coil of wire or winding that is wrapped around a hollow bobbin. There is normally a frame or enclosure made of a magnetically conductive material surrounding the unit. Floating inside the hollow area of the bobbin is a piece of magnetically conductive material designated an actuator or plunger. Typically, the plunger is designed with some sort of feature that allows the user to connect whatever they would need to actuate with it. At one end of the solenoid in the center of the hollow bobbin and attached to the frame is a slug of magnetic material known as a backstop. The backstop dictates the maximum pulling distance the plunger can achieve when the coil is energized. The coil is normally wound to parameters that will dictate the amount of magnetic forces placed upon the plunger based upon an application.
When a solenoid is energized, the magnetic energy causes the plunger to move toward the backstop. As the plunger approaches the backstop, its level of pull force increases. When the plunger bottoms out on the backstop, the maximum amount of pulling power is achieved. Once in this position, the typical application no longer requires the high level of pulling force to hold the plunger in as is needed to pull the solenoid in. Additionally, a solenoid only requires a fraction of the power in the hold in mode as it does for pulling. Since the wattage of the coil does not change, the solenoid simply wastes energy and produces heat when in the holding mode. Such heat can deleteriously affect the performance of the solenoid.
It is desirable to associate the solenoid with a circuit which will monitor source voltage and adjust the PWM duty cycle (ratio of ON time to OFF time) to maintain solenoid coil wattage accordingly. For instance, if a solenoid is designed to operate with a full voltage pull in level of 40 watts at 12 volts and a 5 watt level to hold, using PWM this can be achieved by selecting the proper hold in duty cycle. However, if the voltage should fluctuate to 16 volts, the solenoid would be operating at a value higher than the 5 watt level. Though the holding force would increase, so would the heat rise of the solenoid and potentially create an adverse condition. Likewise, should the source voltage drop to 8 volts, the solenoid would operate far under the 5 watt level and have much reduced holding power. By sampling the source voltage, the PWM duty cycle is increased should the voltage drop, and decreased as the voltage increases. The net result is that whatever the source voltage is, the solenoid coil current will remain constant.
The present invention advantageously provides a pulse width modulated solenoid for use in various applications. In the pulse width modulated solenoid, full voltage is applied to the solenoid to move the actuator from an extended position to a pulled in position. Once the solenoid is in the pulled in position, the voltage is pulse width modulated to reduce the power drawn from the power supply during the hold in state. During the hold in state, the circuit monitors the value of the source voltage and increases or decreases the pulse width based upon the source voltage. In this manner, the control circuit is able to maintain relatively constant power applied to the solenoid to maintain the hold in position.
It is one object of the present invention to provide a versatile, adaptable and highly efficient solenoid which maintains its holding power regardless of various supply voltage.
It is also an object of the present invention to provide a pulse width modulated solenoid with a relatively simple control circuit which will monitor source voltage and adjust duty cycle accordingly.
It is an additional object of the present invention to provide a solenoid with substantially higher pulling force rating which can be used in continuous duty applications. It is another object of the present invention to provide a solenoid having a single winding which is employed for both pull in and hold phases.
It is a further object of the present invention to provide a reduced size solenoid which is designed with adequate pull in and holding forces and is accommodated in confined spaces.
In one aspect of the invention, a circuit controlled, pulse width modulated solenoid is powered by a varying supply voltage. The invention is improved wherein the circuit monitors the varying supply voltage and adjusts the pulse width so that the current flow through the solenoid remains constant.
The invention contemplates a method of controlling a pulse width modulated solenoid powered by a varying supply voltage and having a movable actuator. The method includes the steps of applying power at a first level to move the solenoid actuator from an extended position to a pull in position; and utilizing a pulse generating circuit to define a pulse width mode for applying power at a substantially constant second level less than the first power level to move the solenoid actuator from the pull in position to a hold in position and maintain the hold in position.
In another aspect of the invention, an electromagnetic, pulse width modulated control device includes a solenoid powered by a varying supply voltage and having an actuator movable from an extended position to a pulled in position by a first power level and to a hold in position by a second power level less than the first power level. A pulse width modulated circuit is connected with the solenoid and is responsive to the varying supply voltage for adjusting the pulse width accordingly so as to provide a substantially constant second power level applied to the solenoid to maintain the hold in position. The circuit includes a microcontroller with A/D converter. A voltage regulator is connected between the varying supply voltage and the microcontroller. The microcontroller has an analog input connected to a reference percentage of the varying source voltage obtained through a pair of serially connected resistors. The microcontroller has a digital output connected to a MOSFET which selectively turns the circuit on to allow a substantially constant current flow through the solenoid. A flyback diode is connected in parallel across the solenoid to provide a BACK EMF current path flow in one direction only and prevent burn-out of the MOSFET. The microcontroller also has an internal A/D convertor for converting an analog input voltage into a digital output fed to the MOSFET. A resistor is connected between the voltage regulator and the microcontroller for initiating the microcontroller program sequence.
In yet another aspect of the invention, a control apparatus for regulating a duty cycle type solenoid is powered by a varying supply voltage. A pulse generating means is responsive to the varying supply voltage for generating a driving pulse to the duty cycle type solenoid at a predetermined duty cycle ratio so as to maintain a substantially constant current flow through the solenoid.
Yet another aspect of the invention relates to a method of controlling a pulse width modulated solenoid powered by a varying supply voltage and connected to a pulse width generating control circuit. The method includes the steps of monitoring the supply voltage with the control circuit; and adjusting the pulse width of the control circuit according to the source voltage so as to maintain a substantially constant current flow through the solenoid.